This invention relates to a bridge inverter type X-ray apparatus, and in particular to an X-ray apparatus adopting a secondary winding side feedback control system, which permits feedback to the control system of high-frequency choppers in a bridge inverter by detecting a high voltage output from the secondary winding of a high-tension transformer.
A bridge inverter type X-ray apparatus is adapted to supply an AC output from an AC power source, after passing through a rectifier circuit, to a bridge inverter connected to a primary winding of a high-tension transformer. The bridge inverter is such that four switching elements are connected in a bridge configuration. In this bridge configuration, the two switching elements are connected in a closed circuit including the primary winding of the high-tension transformer and used as high-frequency choppers. When the switching elements are operated in a complementary fashion, high-voltage output is produced from the secondary winding of the high-tension transformer. The high-voltage output is applied to the X-ray tube through the rectifier circuit. It is necessary that the high-voltage output applied to the X-ray tube be stable and free from oscillations. For this reason, a conventional X-ray apparatus, as disclosed in Japanese Patent Application No. 55-108282, adopts what is called a primary winding side feedback system. That is, in the conventional X-ray apparatus, a voltage on the primary winding of the high-tension transformer is detected through a special filter and the detection output is fed back to the high-frequency choppers at a high load time. It has been impossible, however, to perform a feedback control with respect to having loads. A so-called secondary winding side feedback system or a cross regulation system is preferable in the control of high-voltage output applied to the X-ray tube. That is, a voltage on the secondary winding side is detected and the detection voltage is fed back to the control circuit of high-frequency choppers in the bridge inverter.
The secondary winding side feedback system, however, is not adapted for the reason as set out below.
That is, a high-tension cable is used which is shielded between the X-ray tube and a rectifier circuit for rectifying a high-voltage output on the secondary winding side of the high-tension transformer. An electrostatic capacitance is present between the shielded portion and the core conductor of the cable. The inverter elements are alternately conducted due to the coexistence of such electrostatic capacitance with the load impedance and leakage impedance of the high-tension transformer. In the initial portion of an exposure operation by the X-ray tube or when a high-voltage output on the secondary winding side of the high-tension transformer is switched from one polarity to another, "hunting" occurs, causing oscillation of a voltage applied to the X-ray tube and a resultant unstable voltage. "Hunting" also takes place by a possible excessive overshoot occuring during the initial portion of exposure. Even if, at this time, feedback control is effected with respect to the choppers by detecting a tube voltage across the X-ray tube, it has been impossible to obtain a stable voltage waveform to be applied to the X-ray tube.